This invention relates to a method of synthesising a three dimensional sound field.
The processing of audio signals to reproduce a three dimensional sound-field on replay to a listener having two ears has been a goal for inventors for many years. One approach has been to use many sound reproduction channels to surround the listener with a multiplicity of sound sources such as loudspeakers. Another approach has been to use a dummy head having microphones positioned in the auditory canals of artificial ears to make sound recordings for headphone listening. An especially promising approach to the binaural synthesis of such a sound-field has been described in EP-B-0689756, which describes the synthesis of a sound-field using a pair of loudspeakers and only two signal channels, the sound-field nevertheless having directional information allowing a listener to perceive sound sources appearing to lie anywhere on a sphere surrounding the head of a listener placed at the centre of the sphere.
A monophonic sound source can be digitally processed via a xe2x80x9cHead-Response Transfer Functionxe2x80x9d (HRTF), such that the resultant stereo-pair signal contains natural 3D-sound cues as shown in FIG. 1. The HRTF can be implemented using a pair of filters, one associated with a left ear response and the other with a right ear response, sometimes called a binaural placement filter. These sound cues are introduced naturally by the acoustic properties of the head and ears when we listen to sounds in real life, and they include the inter-aural amplitude difference (IAD), inter-aural time difference (ITD) and spectral shaping by the outer ear. When this stereo signal pair is introduced efficiently into the appropriate ears of the listener, by headphones say, then he or she perceives the original sound to be at a position in space in accordance with the spatial location associated with the particular HRTF which was used for the signal-processing.
When one listens through loudspeakers instead of headphones, as is shown in FIG. 2, then the signals are not conveyed efficiently into the ears, for there is xe2x80x9ctransaural acoustic crosstalkxe2x80x9d present which inhibits the 3D-sound cues. This means that the left ear hears a little of what the right ear is hearing (after a small, additional time-delay of around 0.25 ms), and vice versa as shown in FIG. 3. In order to prevent this happening, it is known to create appropriate xe2x80x9ccrosstalk cancellationxe2x80x9d or xe2x80x9ccrosstalk compensationsxe2x80x9d signals from the opposite loudspeaker. These signals are equal in magnitude and inverted (opposite in phase) with respect to the crosstalk signals, and designed to cancel them out. There are more advanced schemes which anticipate the secondary (and higher order) effects of the cancellation signals themselves contributing to secondary crosstalk, and the correction thereof, and these methods are known in the prior art. A typical prior-art scheme (after M R Schroeder xe2x80x9cModels of hearingxe2x80x9d, Proc. IEEE, vol. 63 issue 9, [1975] pp. 1332-1350) is shown in FIG. 4.
When the HRTF processing and crosstalk cancellation are carried out sequentially (FIG. 5) and correctly, and using high quality HRTF source data, then the effects can be quite remarkable. For example, it is possible to move the image of a sound-source around the listener in a complete horizontal circle, beginning in front, moving around the right-hand side of the listener, behind the listener, and back around the left-hand side to the front again. It is also possible to make the sound source move in a vertical circle around the listener, and indeed make the sound appear to come from any selected position in space. However, some particular positions are more difficult to synthesise than others, some it is believed for psychoacoustic reasons, and some for practical reasons.
For example, the effectiveness of sound sources moving directly upwards and downwards is greater at the sides of the listener (azimuth=90xc2x0) than directly in front (azimuth=0xc2x0). This is probably because there is more left-right difference information for the brain to work with. Similarly, it is difficult to differentiate between a sound source directly in front of the listener (azimuth=0xc2x0) from a source directly behind the listener (azimuth=180xc2x0). This is because there is no time-domain information present for the brain to operate with (ITD=0), and the only other information available to the brain, spectral data, is somewhat similar in both of these positions. In practise, there is more high frequency (HF) energy perceived when the source is in front of the listener, because the high frequencies from frontal sources are reflected into the auditory canal from the rear wall of the concha, whereas from a rearward source, they cannot diffract around the pinna sufficiently.
In practical terms, a limiting feature in the reproduction of 3D-sound from two loudspeakers is the adequacy of the transaural crosstalk cancellation, and there are three significant factors here, as follows.
1. HRTF quality. The quality of the 30xc2x0 HRTF (FIG. 3) used to derive the cancellation algorithms (FIG. 4) is important. Both the artificial head from which they derive and the measurement methodology must be adequate.
2. Signal-processing algorithms. The algorithms must be executed effectively.
3. HF effects. In theory, it is possible to carry out xe2x80x9cperfectxe2x80x9d crosstalk cancellation, but not in practise. Setting aside the differences between individual listeners and the artificial head from which the algorithm HRTFs derive, the difficulties relate to the high frequency components, above several kHz. When optimal cancellation is arranged to occur at each ear of the listener, the crosstalk wave and the cancellation wave combine to form a node. However, the node exists only at a single point in space, and as one moves further away from the node, then the two signals are no longer mutually time-aligned, and so the cancellation is imperfect. For gross misalignment, then the signals can actually combine to create a resultant signal which is greater at certain frequencies than the original, unwanted crosstalk itself. However, in practise, the head acts as an effective barrier to the higher frequencies because of its relative size with respect to the wavelengths in question, and so the transaural crosstalk is limited naturally, and the problem is not as bad as might be expected.
There have been several attempts to limit the spatial dependency of crosstalk cancellation systems at these higher frequencies. Cooper and Bauck (U.S. Pat. No. 4,893,342) introduced a high-cut filter into their crosstalk cancellation scheme, so that the HF components ( greater than 8 kHz or so) were not actually cancelled at all, but were simply fed directly to the loudspeakers, just as they are in ordinary stereo. The problem with this is that the brain interprets the position of the HF sounds (i.e. xe2x80x9clocalisesxe2x80x9d the sounds) to be where the loudspeakers themselves are, because both ears hear correlating signals from each individual speaker. It is true that these frequencies are difficult to localise accurately, but the overall effect is nevertheless to create HF sounds of frontal origin for all required spatial positions, and this inhibits the illusion when trying to synthesise rearward-positioned sounds.
Even when the crosstalk is optimally cancelled at high frequencies, the listener""s head is never guaranteed to be exactly correctly positioned, and so again, the non-cancelled HF components are xe2x80x9clocalisedxe2x80x9d by the brain at the speakers themselves, and therefore can appear to originate in front of the listener, making rearward synthesis difficult to achieve.
The following additional practical aspects also hinder optimal transaural crosstalk cancellation:
1. The loudspeakers often do not have well-matched frequency responses.
2. The audio system may not have well-matched L-R gain.
3. The computer configuration (software presets) may be set so as to have inaccurate L-R balance.
Many sound sources which are used in computer games contain predominantly low-frequency energy (explosion sounds, for example, and xe2x80x9ccrashxe2x80x9d effects), and so the above limitations are not necessarily serious because the transaural crosstalk cancellation is adequate for these long wavelength sources. However, if the sound sources were to contain predominantly higher-frequency components, such as bird-song, and especially if they comprise relatively pure sine-wave type sounds, then it would be very difficult to provide effective crosstalk cancellation. Bird-song, insect calls and the like, can be used to great effect in a game to create ambience, and it is often required to position such effects in the rearward hemisphere. This is particularly difficult to do using presently known methods.
According to the present invention there is provided a method of synthesising a three dimensional sound-field using a system including a pair of front loudspeakers arranged in front of a preferred position of a listener and a pair of rear loudspeakers arranged behind said preferred position, including:
a) determining the desired position of a sound source in said three dimensional sound-field relative to said preferred position;
b) providing a binaural pair of signals comprising a left channel and a right channel corresponding to said sound source in said three dimensional sound field;
c) controlling the gain of the left channel signal of said binaural pair of signals using a front signal gain control means and a rear signal gain control means to provide respective gain controlled front left and rear left signals respectively;
d) controlling the gain of the right channel signal of said binaural pair of signals using a front signal gain control means and a rear signal gain control means to provide respective gain controlled front right and rear right signals respectively;
e) controlling the ratio of the front signal pair gain to the rear signal pair gain as a function of the desired position of said localised sound source relative to said preferred position; and
f) performing transaural crosstalk compensation on the gain controlled front signal pair and rear signal pair using respective transaural crosstalk compensation means, and using these two compensated signal pairs to drive the corresponding loudspeakers in use.
The present invention relates to the reproduction of 3D-sound from multiple-speaker stereo systems, and especially four-speaker systems, for providing improved effectiveness of rearward placement of virtual sound sources. Whereas present two-loudspeaker 3D-sound systems are advantageous over multi-speaker systems for the obvious reasons of cost, wiring difficulties and the need for extra audio drivers, the present invention takes advantage of the fact that a proportion of multi-media users will already possess, or will buy, a 4 (or more) speaker configuration to cater for alternative formats, such as Dolby Digital(trademark). (Note, however, that such formats are only 2D xe2x80x9csurroundxe2x80x9d systems, incapable of true 3D source placement, unlike the present invention.) The present invention enables conventional, two-speaker, 3D-sound material to be replayed over such four (or more) speaker systems, to provide true, 3D virtual source placement. The invention is especially valuable in rendering effective rearward placement of virtual sound sources which are rich in HF (high frequencies), thus providing enhanced 3D-sound for the listener. This is achieved in a very simple, but effective, way.
First, for descriptive reasons, it is useful to establish a spatial reference system with respect to the listener, as is shown in FIG. 12, which depicts the head and shoulders of a listener surrounded by a unit-dimension reference sphere.
The horizontal plane cutting the sphere is shown in FIG. 12, together with the horizontal axes. The front-rear axis is P-Pxe2x80x2, and the lateral axis is Q-Qxe2x80x2, both passing through the centre of the listener""s head. The convention chosen here for referring to azimuth angles is that they are measured from the frontal pole (P) towards the rear pole (Pxe2x80x2), with positive values on the right-hand side of the listener and negative ones on the left-hand side. For example, the right-hand side pole, Qxe2x80x2, is at an azimuth angle of +90xc2x0, and the left-hand pole (Q) is at xe2x88x9290xc2x0. Rear pole Pxe2x80x2 is at 30 180xc2x0 (and xe2x88x92180xc2x0). The median plane is that which bisects the head of the listener vertically in a front-back direction (running along axis P-Pxe2x80x2). Angles of elevation are measured directly upwards (or downwards, as appropriate) from the horizontal plane.
In principle, a two-channel 3D-sound signal can be replayed effectively through either (a) a frontal pair of speakers (xc2x130xc2x0); (b) a rearward pair of speakers (xc2x1150xc2x0), as described in GB 2311706 B; or (c) both of these simultaneously. However, when the crosstalk cancellation is caused to be less than fully effective, for reasons described previously, such as poor L-R balance, then the virtual sound images are either moved towards the loudspeaker positions, or xe2x80x9csmeared outxe2x80x9d between their location and the speakers. In extreme conditions, the image can break down and be unclear. The following two examples illustrate the point.